Voltage regulators are known of the type comprising: an input for an electric control signal which indicates the desired speed of the motor, and a control circuit device with pulse width modulation (PWM), which is connected between a direct-current supply voltage source and the motor and which is capable of applying to the motor a square wave voltage which is obtained from the voltage produced by that source and which has a duty cycle which varies in accordance with a predetermined function of that control signal.
Conventionally, in order to vary the rotation speed of two electric fans on board motor vehicles, there are used two separate electronic regulating devices, each one associated with a motor of an electric fan, respectively. In that manner, the two motors are controlled in a completely independent manner. Advantageously, those electronic regulating devices are integrated in a single voltage control and regulating device.
FIG. 1 is a partially schematic electric circuit diagram of a voltage control and regulating device according to the above-described prior art.
In particular, a first and a second direct-current electric motor are designated M1 and M2, respectively, such as electric fan motors for a radiator of an internal-combustion engine in a motor vehicle.
Each motor M1 and M2 is connected to a voltage control and regulating device which is generally designated 1. That device 1 is connected to a direct-current supply voltage source 2, such as a battery, and comprises two electronic regulating devices which are generally designated 4a and 4b. Each electronic regulating device 4a and 4b is connected to the first and second motor M1 and M2 by means of a connection branch 14a and 14b, respectively.
Each electronic regulating device 4a and 4b comprises: an input 8a, 8b for an electric control signal which indicates the desired speed for the associated motor M1, M2; a control circuit device with pulse width modulation (PWM) which is generally designated 6a and 6b, respectively; and a control module 10a, 10b which is capable of supplying to the associated PWM control device 6a, 6b a conditioning signal in such a manner that it applies to the motor M1, M2 a square wave voltage obtained from the voltage VB produced by the battery 2, respectively, and having a duty cycle DC which varies in accordance with a predetermined function of the electric control signal applied to the input 8a, 8b of the electronic regulating device 4a and 4b, respectively.
In a manner known per se, each PWM control device 6a, 6b comprises at least one electronic solid state switch, such as a MOSFET transistor Q1, Q2 which is controlled by the associated control module 10a, 10b. 
Each PWM control circuit device 6a, 6b is connected at one side to the positive pole of the voltage source 2 and to the earth GND at the other side.
Advantageously, in series with each PWM control circuit device 6a, 6b, there is provided at the side of the earth GND a device 12a, 12b, respectively, for controlling the polarity of the voltage source 2 which is provided to prevent inversion of the polarity thereof. Preferably, those control devices 12a, 12b each comprise a MOSFET transistor (not shown in the Figure) in a manner known per se.
Advantageously, in series with each motor M1, M2, there is provided a shunt resistor Rsa, Rsb which is capable of allowing, in a manner known per se, monitoring of the current flowing in the motor M1, M2, respectively, and therefore the correct operation thereof. Although the shunt resistors Rsa, Rsb are illustrated outside the regulators 4a, 4b for the sake of simplicity, they belong conceptually to the regulators and may be constructed inside them.
There is associated with each electronic regulating device 4a, 4b, in a manner known per se, a filtering circuit (not shown in the Figure) which is capable of ensuring the correct operation of the associated PWM control circuit device 6a, 6b. 
The solution described with reference to FIG. 1 has the disadvantage of being costly because it requires substantial duplication of all the electronic devices. In particular, two filtering circuits which are costly and complex to construct are necessary.
The above-mentioned disadvantages have been overcome by adopting a single electronic voltage regulator which controls two separate motors which are positioned in parallel between the voltage source 2 and the earth GND.
FIG. 2 illustrates a partially schematic electric circuit diagram of an electronic voltage regulating device according to the additional prior art set out above. Elements which are similar to those illustrated in FIG. 1 have been indicated with the same reference numerals.
The two motors M1 and M2 which are positioned substantially parallel with each other are then connected by means of a single connection branch 14 to the electronic regulating device 4.
In the configuration of FIG. 2, a reduction in the costs is obtained because the number of devices used is reduced; in particular, a single filtering circuit is sufficient.
However, the possibility of controlling each motor in a manner independent of the other is lost and therefore the possibility of independently controlling any failures which are established in one of the two motors is also lost. This means that, if one motor fails, the other can also no longer be used.
An object of the present invention is to provide a solution which allows the construction of a voltage regulator in which it is possible to independently control the failures which may be established in each motor and which is simultaneously less expensive than the above-discussed prior art.